The present invention relates to an improved apparatus and process for the removal of sulfur oxide from sulfur oxide-containing gases, and more particularly, to an improved apparatus and process for the simultaneous absorption of sulfur dioxide and production of ammonium sulfate from gas containing sulfur dioxide.
Sulfur dioxide emission control from such sources as fossil fuel fired boilers, smelters, sulfuric acid plants, pulp and paper mill operations and the like is required by law in many countries to mitigate the serious environmental and health damage that is associated with sulfur dioxide. The most widely practiced method for sulfur dioxide control is based upon limestone or lime contact with flue gases in the form of aqueous slurry. In most instances, the by-product is either discarded as a land fill or converted into gypsum for use in wall board and cement manufacture. In a few instances, other alkaline reagents, such as sodium, magnesium and ammonia have been used with recovery of useful by-products, such as pure sulfur dioxide, sulfuric acid and sulfur.
Use of ammonia and ammoniacal scrubbing solutions for flue gas desulfurization is also well known. Ammonium sulfate is often formed by contact between sulfur oxide containing flue gases and ammoniacal solutions, the ammonium sulfate being formed as crystals and precipitated from solution. In Defensive Publication No. T909,017, sulfur dioxide is removed from stack gases by absorption in ammoniacal solution forming ammonium bisulfite, ammonium sulfite and, unavoidably, ammonium sulfate. The absorber effluent solution may be treated with ammonium bisulfate to form ammonium sulfate and to release a concentrated stream of sulfur dioxide suitable for conversion to sulfuric acid or elemental sulfur. Ammonium sulfate may be crystallized from the solution and thermally decomposed to ammonia for recycle to the absorber and ammonium bisulfate for recycle to the sulfur dioxide release step.
In U.S. Pat. No. 3,186,802, sulfur oxides are removed from waste or flue gas by contact of the gas with ammonia. In a reaction vessel in U.S. Pat. No. 3,186,802, an incoming acidified feed liquor is mixed with a recirculated sulfuric acid containing stream producing a reaction in which ammonium sulfite and ammonium bisulfite are converted to ammonium sulfate and sulfur, and small amounts of sulfuric acid are used tc scrub trace amounts of ammonia from the flue gas.
In still another process for the removal of sulfur dioxide from flue gas by contact of the gas with an ammonia solution, flue gas is introduced into the lower portion of a column while the ammonia solution is introduced into the upper portion of the column, the ammonia reacting with the sulfur dioxide to produce ammonium sulfite. This process as described in U.S. Pat. No. 3,843,789 removes the sulfur dioxide to reduce air pollution.
Ammonium sulfate is produced during the separation of sulfur dioxide from a gaseous stream by contacting the gaseous stream with ammonia in the presence of oxygen and water in a multiple stage liquid-vapor contactor or contact device comprised of at least three sections or zones as disclosed in U.S. Pat. No. 4,250,160. In U.S. Pat. No. 4,250,160, in the contactor, in a central section, sulfur dioxide is contacted in vapor phase with an excess of ammonia in the presence of oxygen and water vapor to produce ammonium sulfate. In an upper section of the contactor, excess ammonia is scrubbed from the effluent gas by countercurrent contact with water or acid solution. Liquid cyclones are utilized in the lower section of the contactor, and ammonium sulfate in crystalline form is removed from the lower section of the contactor. A slurry of ammonium sulfate is removed from the bottom stages of the contactor, crystalline ammonium sulfate is separated from the slurry, and the mother liquor is recycled to the contactor. However, the primary purpose of the central section in U.S. Pat. No. 4,250,160 is to provide vapor phase contact between the sulfur dioxide and ammonia, the sulfur dioxide being reacted with an excess concentration of ammonia in the presence of excess oxygen and water vapor to produce ammonium sulfate.
Ammonium sulfate is prepared by introducing an oxidizing gas into a solution containing ammonium sulfite in a closed chamber and by building up the pressure therein to convert a major portion of the sulfite to sulfate, and thereafter discontinuing the introduction of the oxidizing gas and converting the remainder of the sulfite to sulfate by recirculating the gas contained therein under the built-up pressure through the solution in U.S. Pat. No. 1,986,889. In U.S. Pat. No. 1,986,889, the alkalinity or acidity of the ammonium sulfite solution in the column may be controlled by admitting ammonia-containing gas or sulfur dioxide-containing gas into the solution. To prevent deposition of solid ammonium salt after ammonia has been fed into the solution, the ammonia gas is swept out by the oxidizing gas before admitting sulfur dioxide gas, and sulfur dioxide is scrubbed out before ammonia can be admitted.
All of the above processes and devices have been found to be inadequate or inefficient for the removal of sulfur oxides from sulfur oxide-containing gases. Generally, the large scale use of ammonia for flue gas desulfurization has not been commercially practiced due to basic problems. The first problem is the relative expense of ammonia. Because of the expense of ammonia, it must be recycled with sulfur extraction or converted to ammonium sulfate for use as a fertilizer. The current state of the art makes both of these options rather complex to practice, and the commercial plants to carry out the processes are relatively expensive to build. Secondly, ammonia gas tends to escape into flue gases during desulfurization leading to not only economic loss but also generating a visible blue fume in the stack discharge, thus creating a secondary pollution problem. In order to overcome this disadvantage and to control ammonia loss and blue fume, rather complex scrubbing systems which are expensive to operate, have been proposed.